Can the realism of high-resolution models be gauged by comparing simulated with observed KE spectra?
Abstract
It is often argued that high-resolution models are better than lower-resolution models because they are better at capturing the relatively flat -5/3 slope of the mesoscale KE spectrum reported in Nastrom and Gage's 1986 study of upper tropospheric aircraft observations. In light of the vastly greater number of observations that have been analyzed since that time, we have revisited the question of what is the slope of the "true" spectrum. Specifically, we have examined the 200-hPa KE spectra in several high-resolution global reanalysis datasets, including NCEP GFS (resolution T1534 and T254), ERA-Interim (T255), ERA5 (T639), and JRA-55 (T319). We found that the mesoscale portions of the global spectra are highly mutually inconsistent. This is primarily because the global mesoscale KE has a large contribution from the KE in convective regions, which differs greatly among the various reanalyses.
In addition to this lack of consensus on the slope of the "true" spectrum, there is also some sensitivity of the slope of a model's simulated spectrum to its diabatic and horizontal diffusion parameterizations. We assessed this sensitivity in two ways: 1) by stochastically perturbing the physical tendencies and 2) by decreasing the hyper-viscosity coefficient, in large ensembles of 15-day forecasts made using the NCEP GFS (T254) model. Both changes increased the mesoscale KE and decreased the steep spectral slope. The effect was stronger in the modified hyper-viscosity experiment. We thus conclude that (a) high-resolution global reanalyses do not yet converge to a "true" mesoscale KE spectrum; (b) the KE spectra in models are sensitive to and can be manipulated by perturbing the physics and tuning the horizontal diffusion, besides increasing model resolution; and (c) given the lack of consensus on the slope of the "true" spectrum, increasing model resolution to pursue the -5/3 slope of the Nastrom-Gage spectrum may be misleading.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.A11Q2744W
- Keywords:
-
- 3337 Global climate models;
- ATMOSPHERIC PROCESSES;
- 1622 Earth system modeling;
- GLOBAL CHANGE;
- 1627 Coupled models of the climate system;
- GLOBAL CHANGE;
- 1655 Water cycles;
- GLOBAL CHANGE